This is mostly a proof of concept/RFC patch. The idea is for
git-send-email to store the message-ids it generates, along with the
Subject and Date headers of the message. When prompting for which
Message-ID should be used in In-Reply-To, display a list of recent
emails (identifed using the Date/Subject pairs; the message-ids
themselves are not for human consumption). Choosing from that list
will then use the corresponding message-id; otherwise, the behaviour
is as usual.